1. Field of the Invention
This invention relates to photography and, more particularly, to photoelectric photometers such as the unicell photometer for use in exposure control applications.
2. Background of the Prior Art
The use of unicell photometers for evaluating scene luminances to predict exposures is well known in the prior art.
These devices generally consist of a photosensitive detector and an optical system for collecting radiant energy from a scene and directing it onto a surface of the photosensitive detector. The detector is usually a photoelectric transducer which integrates the total energy incident on its surface and converts it to an electric signal. The strength of the signal is used as an indication of what the exposure should be, based on the sensitometric characteristics of the film being used and what the photographer regards as his subject.
For anyone who must use or design them, the exact meaning of what the single output of the detector represents, in terms of its relationship to scene characteristics, is one of the most troublesome aspects of the unicell photometer. To understand this statement, consider what takes place when a unicell photometer is aimed at a scene to be photographed. The scene, in general, includes a number of objects, luminance sources, scattered throughout it in a more or less random pattern. The optical system of the photometer collects all the radiation from all of these objects within its field of view. The detector, in turn, responds to the radiation from each of the sources by summing up the individual responses that can be attributed to each object. The result is a single electrical signal which may or may not be interpreted as representing an average scene brightness for purposes of predicting the exposure. However, even if it is interpreted as being the average, that decision may not be appropriate if those objects that are considered to be the subject substantially differ in brightness from the average, or worse, fall outside the response capability of the film.
In spite of this fundamental drawback, the unicell photometer still remains attractive because of its simplicity and cost. Moreover certain steps can be taken to minimize errors by carefully analyzing those factors that produce them. For example, it has been found that the performance of the unicell photometer as an exposure predictor or discriminator is a strong function of its directional response. Therefore, by controlling the directional response, it is possible to improve exposure prediction capability if certain assumptions are made concerning the probability of occurrence of the distribution of object luminances, both in space and time, for typical scenes. With both these factors known, the directional response and the scene luminance distribution, to a degree of certainty, the designer is in a position to begin to make more sense out of what the single detector output represents.
For example, if the relative response of the detector to an object located ten degrees off axis is, say, 30 percent of its on-axis response, and the probability of an important object being located in the scene at that location is high, that object is very likely going to be misexposed given the fact that most subjects are predominantly near the central axis. Under these conditions, correction to the directional response pattern may be well advised.
The implication being made here is not what the directional response should be; there seems to be a fair amount of controversy regarding that issue in the industry. Each manufacturer has his own preference based on his own research into these matters. The important point is to recognize that the directional response pattern can be used to significantly improve exposure prediction performance in unicell photometers. Unfortunately, the directional response patterns of unicell photometers assume rather characteristic shapes once their geometric configuration is established, and the degree to which those shapes can be altered by manipulating design parameters is limited. Consequently, any reduction in the available design degrees of freedom greatly hampers a designer's ability to achieve a particular response pattern.
Usually the parameters that influence the response pattern are detector size, local surface sensitivity of the detector, the optics of the collecting system, and the distance separating the detector and optical system.
The present invention is concerned with a predetector optical system that may be used to successfully achieve a preferred directional response pattern when the distance from the detector to the optical system is a design constraint. In addition, the transfer function, or the amount of energy collected by the optical system compared to the amount available for collection, is maximized.